For example, the main clutch of a farm tractor is provided between a flywheel mounted on the crankshaft of an engine and a propeller shaft rotatably supported by the wall of a clutch housing. A release hub mounted on the propeller shaft is axially shifted by depressing a clutch pedal to thereby cause a release bearing on the hub to push clutch disengaging members, whereby the clutch is disengaged (see, for example, Unexamined Japanese Utility Model Publication SHO 57-9625).
With the clutch operating device described, the depression of the clutch pedal is transmitted through an arm, and a link or wire to a clutch release fork, which in turn pushes the release hub.
The conventional clutch operating device necessitates many parts, such as arm, link or wire, clutch release fork and pivot, is complex in construction, is not fully resistant to great loads and requires a great force to depress the pedal.
Accordingly, the specification of U.S. Pat. No. 3,687,253, for example, discloses a device adapted to operate a clutch with a small fluid pressure instead of the above mechanical means.
This conventional clutch operating device of the fluid pressure type has a stationary sleeve fitted around a propeller shaft concentrically therewith and projecting from the support wall of a transmission case toward the clutch. The stationary sleeve has a peripheral shoulder, a reduced diameter end portion closer to the clutch, and a large diameter base portion adjacent to the support wall. A movable sleeve is slidably fitted around the reduced diameter portion of the stationary sleeve. The movable sleeve carries a release bearing, which is engageable with clutch disengaging members. A collar is axially slidably fitted around the large diameter portion of the stationary sleeve and around one end portion of the movable sleeve. The collar is axially shiftable by the pivotal movement of a yoke, which is coupled to a clutch pedal. The shoulder of the stationary sleeve, the end face of the movable sleeve and the inner surface of the collar define a fluid chamber, to which a fluid is supplied by a pump. The fluid supplied to the fluid chamber flows through a clearance between the end portion of the movable sleeve and the inner surface of the collar and is forced out from passages communicating with the outside and sprayed on the clutch disk to cool the clutch.
To disengage the clutch with use of the conventional operating device of the fluid pressure type, the yoke is pivotally moved by depressing the clutch pedal, whereby the collar is shifted toward the movable sleeve. The shift of the collar diminishes the clearance of the fluid chamber formed between the movable sleeve and the collar inner surface to increase the fluid pressure within the fluid chamber and push the movable sleeve with the pressure. The movable sleeve in turn pushes the clutch disengaging members to disengage the clutch.
However, the conventional device is not usable for disk clutches of the dry type since it is adapted to force out the fluid from the fluid passages and cool the disk at all times.
Further although the conventional clutch operating device of the fluid pressure type is adapted to engage and disengage the clutch by depressing the clutch pedal, the operator is unable to detect deliberate slipping of the clutch from the resistance to the depressing action.
With the aforementioned mechanical clutch operating device, the pressure of the clutch disk can be sensed as resistance to depression, so that the clutch in the state of deliberate slipping can be detected by the foot for delicate manipulation. However, with the conventional fluid pressure-type device wherein the collar is shifted to merely control the clearance, the operator is unable to sense the clutch slipping with his foot and therefore experiences difficulty in manipulating the clutch delicately.
Further with the fluid pressure device wherein the yoke is manipulated as in the mechanical device, the clutch pedal must be coupled to the yoke with a wire, rod or the like. It is therefore impossible to simplify the construction.